How do I blend textures of joined objects?
If you can add an image of the wireframe, that would be helpful.

The short answer is that you'll need to make the mesh for the bullet (top) and the casing (bottom) connected. Once you do that, there will be a less noticeable seam.

That having been said, normal ammunition has a very distinct seam between the bullet and the casing. If you make the bullet orange-ish and the casing yellow-ish, it'll look like a real bullet.

If that's not what you're after, then you should explore how to model the bullet and the casing together as one mesh. One approach might be to delete the bottom half of the spherical bullet (the part that I'm assuming is hidden inside the casing). Then extrude the bottom ring downward into a cylinder. Then close the cylinder at the bottom. This will result in a bullet and casing that are one piece, and might make your texturing job easier.